not known how nonhuman primates, consuming only plant material, obtain this vitamin, but it is possible that vitamin B₁₂ is synthesized by microorganisms in the gastrointestinal tract (Uphill et al., 1977). Primates that practice coprophagy may obtain vitamin B₁₂ from ingested feces (Oxnard, 1989). Little is known about the biologic availability of vitamin B₁₂ in natural food ingredients (Baker, 1995). Supplemental vitamin B₁₂ is usually added to animal feeds as cyanocobalamin.

The signs of vitamin B₁₂ deficiency in humans are megaloblastic anemia and progressive demyelination and neuropathy (Herbert, 1996). A frank deficiency of vitamin B₁₂ was produced under controlled conditions in rhesus monkeys (Macaca mulatta) by feeding a purified diet containing soy protein rather than casein to avoid potential contamination by vitamin B₁₂ in the latter. During the first 12-18 months, serum concentrations of B₁₂ dropped to 5-10% of initial values. Liver vitamin B₁₂ concentrations were less than 5% of those in supplemented animals. Methlymalonic acid concentrations in the urine, a biochemical indicator of deficiency, increased in deficient animals but not in supplemented controls. In spite of the apparent depletion of vitamin B₁₂ stores, no other manifestations of deficiency were seen (Kark et al., 1974). The studies were then extended. Monkeys fed the deficient diet for a total of 33-45 months exhibited additional deficiency signs, including visual impairment that gradually progressed to blindness, spastic paralysis of the hind limbs and tail, general weakness, apathy, and death. At necropsy, degeneration of nervous tissue was evident, with eventual destruction of the myelin sheath and loss of axons (Agamanolis et al., 1976, 1978; Chester et al., 1980). The degeneration of central nervous tissue was similar to “subacute combined degeneration,” one of the clinical diseases seen in human vitamin B₁₂ deficiency. Even in the most severe cases of vitamin B₁₂ deficiency, no signs of anemia or any other blood disorders were observed.

Chronic deficiency of vitamin B₁₂ in nonhuman primates under somewhat less controlled conditions has also been described. Blood concentrations of vitamin B₁₂ in newly